In recent years, lithographic techniques have evolved and have been diversified more and more, and various exposure methods have been proposed as emerging lithographic techniques attempting new possibilities.
Among them, U.S. Pat. No. 6,171,730 discusses an example that proposes a near-field exposure method for performing near-field exposure based on a contact method, in comparison with conventional projection exposure methods.
As the light blocking material of a mask to be used in such a contact near-field exposure method, use of silicon has been suggested (The 53rd Japanese Applied Physics Alliance Lecture Proceedings, 25a-ZB-1, 2006, Spring).
On the other hand, Japanese Laid-Open Patent Application No. 2005-101133 proposes that, after a mask and a wafer are closely contacted and they are exposed, if there is an attraction force based on static electricity working therebetween to attract them, an outside mechanism that neutralizes the electrification is used to weaken the electrostatic attraction force in order to facilitate separation of the mask and the wafer.
Furthermore, Japanese Laid-Open Patent Application No. 2000-321777 proposes an exposure apparatus wherein static electricity produced is removed by projecting soft X-rays to a photomask, in a direction along the mask surface, thereby to prevent adhesion of dust or foreign particles on the photomask or the wafer.
The near-field exposure process can provide a resolving power that enables patterning of a half pitch size of 45 nm or less. In order to meet this, very high precision is required for the alignment between the exposure mask and the substrate.
The near-field exposure method proposed by U.S. Pat. No. 6,171,730 is a method in which close contact, exposure and separation of an exposure mask and a substrate coated with a resist are carried out repeatedly. Therefore, if a material having a low electrical conductivity is used as the light blocking material, static electricity will be produced easily.
On the other hand, the near-field exposure method described in the 53rd Japanese Applied Physics Alliance Lecture Proceedings, 25a-ZB-1, 2006, Spring, uses amorphous silicon having a low electrical conductivity as a light blocking material, which can be easily electrically charged, as compared with metal light blocking films conventionally being used.
Additionally, electrification may be caused by vibration of the machine during mask conveyance or mask loading/unloading.
Due to the static electricity produced between the exposure mask and the substrate being charged, as described above, an attraction force or a repulsive force is produced, and applied between them.
In the exposure method based on near-field light, the near-field light exists only in a limited region of about 100 nm or less, which is extraordinarily close to the exposure mask surface. Therefore, it is necessary to perform the exposure while keeping the exposure mask and the resist-coated substrate surface in close contact with each other.
Thus, if the alignment operation for the exposure mask and the resist-coated substrate surface is carried out after they are brought into close contact with each other, it means that the relative position of the mask and the substrate is changed, while they are kept in close contact with each other. This causes several problems, such as resist exfoliation or mask breakage, for example.
In order to avoid this, for alignment of an exposure mask and a resist-coated substrate, the alignment operation for them should be carried out before they are brought into close contact with each other.
However, if the exposure mask is made of an elastic material, which is very thin in thickness, as described in U.S. Pat. No. 6,171,730, or of a material which itself is soft, like a resin, there is a possibility that the exposure mask is deformed by the electrostatic force.
For example, if, as shown in FIG. 2A, an exposure mask 101 and a resist-coated substrate 102 are locally charged, the portions of the mask and the substrate, where a stronger attraction force is applied, may be strongly attracted to each other, as shown in FIG. 2B, to cause deflection of the exposure mask.
As described above, when the exposure mask 101 or the resist-coated substrate 102 is being electrically charged, even if the alignment operation for the exposure mask and the resist-coated substrate is completed while they are separated from each other (that is, before they are brought into close contact with each other), nevertheless, the exposure mask distortion will be caused by the electrostatic force in the middle of the approaching motion for closely contacting the mask and the substrate. If this occurs, the position of the exposure mask may deviate from the position where the mask has been aligned with the substrate, while being kept apart from the substrate.
Furthermore, distortion of the exposure mask causes distortion of the mask pattern, which interrupts accurate pattern exposure.
Additionally, if a mask is distorted by an electrostatic force, even though it occurs before carrying out the alignment process, the position of alignment marks of the exposure mask may deviate, which, in turn, interrupts the alignment operation itself.
Japanese Laid-Open Patent Application No. 2005-101133 shows a structure in which removal of the electrification is carried out only when an exposure mask and a resist -coated substrate are just going to be separated from each other. In other words, it is not a structure configured so that the removal of the electrification is carried out before the alignment operation to remove the electrification of the exposure mask and the resist-coated substrate.
Furthermore, in Japanese Laid-Open Patent Application No. 2000-321777, although soft X-rays are projected in a direction along the surface of the exposure mask and the resist-coated substrate to remove electrification of them, the removal of the electrification effect based on the soft X-rays decreases largely in the direction of irradiation.
Therefore, in order to complete the removal of the electrification throughout the whole surface, it is necessary to wait until the farthest point from the soft X-ray projector to have the electrification removed.
Furthermore, if the method disclosed in Japanese Laid-Open Patent Application No. 2000-321777, mentioned above, is applied to the exposure method described in U.S. Pat. No. 6,171,730, wherein a thin-film mask is deflected and contacted to a substrate, there will be inconveniences, such as follows. Namely, if the soft X-rays are projected along the mask surface, there is a possibility that the thin-film mask being flexed blocks the radiation. If this occurs, it causes uneven removal of the electrification of the exposure mask and the resist-coated substrate.